The present invention relates generally to gas anesthesia machines, and more particularly, to alarm systems for use in conjunction with gas anesthesia machines, to detect a loss of oxygen pressure.
A gas anesthesia machine typically comprises an oxygen supply circuit and an anesthesia gas supply circuit, for anesthesia gas such as nitrous oxide (N.sub.2 O), a gas mixing circuit wherein oxygen and anesthesia gas are mixed together and a common outlet through which a mixture of the gases is passed to a patient breathing circuit. The patient breathing circuit, which forms no part of the present invention, is ordinarily a closed-circuit system including a carbon dioxide absorber and at least two check valves, to ensure that a patient inhales gas from the common outlet and exhales through the carbon dioxide absorber. The patient breathing circuit may also employ a ventilator to pump breathing gas into the patient's lungs and a gas evacuation system for removal of excess gas.
The oxygen supply circuit of the anesthesia machine is pressurized to ensure the delivery of a proper flow of oxygen to a patient. The maintenance of the oxygen flow above a critical minimum level is important to protect the patient from injury from oxygen deprivation.
Occasionally, pressure within the oxygen supply circuit may fall below the critical level. For example, a malfunction in the oxygen supply circuit, such as a leak, may lead to a pressure drop within the circuit and a corresponding decrease in oxygen flow to the patient. As a result, the patient is in danger of serious injury unless an operator of the gas anesthesia machine quickly restores a safe oxygen flow. Although other oxygen pressure loss alarms have been used in the past, none has been completely satisfactory in all respects. Ideally, an oxygen pressure loss alarm should be pneumatically operated, for reliability, and should be responsive not only to loss of pressure due to duration of the oxygen supply, but also to pressure loss that occurs upon shutting off the gas anesthesia machine. The present invention is directed to these ends.